Origin of Key Cosmic Explosions Still a Mystery

July 12, 2010

This Hubble image reveals the gigantic Pinwheel Galaxy (M101), one of the best known examples of "grand design spirals," and its star-forming regions in unprecedented detail. Astronomers have searched galaxies like this in a hunt for the progenitors of Type Ia supernovae, but their search has turned up mostly empty-handed. Credit: NASA/ESA

(PhysOrg.com) -- When a star explodes as a supernova, it shines so brightly that it can be seen from millions of light-years away. One particular supernova variety - Type Ia - brightens and dims so predictably that astronomers use them to measure the universe's expansion. The resulting discovery of dark energy and the accelerating universe rewrote our understanding of the cosmos. Yet the origin of these supernovae, which have proved so useful, remains unknown.

"The question of what causes a Type Ia supernova is one of the great unsolved mysteries in astronomy," says Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics (CfA).

Astronomers have very strong evidence that Type Ia supernovae come from exploding stellar remnants called white dwarfs. To detonate, the white dwarf must gain mass until it reaches a tipping point and can no longer support itself.

There are two leading scenarios for the intermediate step from stable white dwarf to supernova, both of which require a companion star. In the first possibility, a white dwarf swallows gas blowing from a neighboring giant star. In the second possibility, two white dwarfs collide and merge. To establish which option is correct (or at least more common), astronomers look for evidence of these binary systems.

Given the average rate of supernovae, scientists can estimate how many pre-supernova white dwarfs should exist in a galaxy. But the search for these progenitors has turned up mostly empty-handed.

To hunt for accreting white dwarfs, astronomers looked for X-rays of a particular energy, produced when gas hitting the star's surface undergoes nuclear fusion. A typical galaxy should contain hundreds of such "super-soft" X-ray sources. Instead we see only a handful. As a result, a recent paper suggested that the alternative, merger scenario was the source of Type Ia supernovae, at least in many galaxies.

That conclusion relies on the assumption that accreting white dwarfs will appear as super-soft X-ray sources when the incoming matter experiences nuclear fusion. Di Stefano and her colleagues have argued that the data do not support this hypothesis.

In this negative image of the Pinwheel Galaxy (M101), red squares mark the positions of "super-soft" X-ray sources. The Pinwheel should contain hundreds of accreting white dwarfs on which nuclear fusion is occurring, which should produce prodigious X-rays. Yet we only detect a few dozen super-soft X-ray sources. This means that we must devise new methods to search for the elusive progenitors of Type Ia supernovae. Credit: R. Di Stefano (CfA)

In a new paper, Di Stefano takes the work a step further. She points out that a merger-induced supernova would also be preceded by an epoch during which a white dwarf accretes matter that should undergo nuclear fusion. White dwarfs are produced when stars age, and different stars age at different rates. Any close double white-dwarf system will pass through a phase in which the first-formed white dwarf gains and burns matter from its slower-aging companion. If these white dwarfs produce X-rays, then we should find roughly a hundred times as many super-soft X-ray sources as we do.

Since both scenarios - an accretion-driven explosion and a merger-driven explosion - involve accretion and fusion at some point, the lack of super-soft X-ray sources would seem to rule out both types of progenitor. The alternative proposed by Di Stefano is that the white dwarfs are not luminous at X-ray wavelengths for long stretches of time. Perhaps material surrounding a white dwarf can absorb X-rays, or accreting white dwarfs might emit most of their energy at other wavelengths.

If this is the correct explanation, says Di Stefano, "we must devise new methods to search for the elusive progenitors of Type Ia supernovae."

Di Stefano's paper has been accepted for publication in The Astrophysical Journal and is available online.

Related Stories

Scientists from the University of Hertfordshire have discovered a rare binary system consisting of a white dwarf, a Sun-like star that has reached the end of its life, and an ultra-cool dwarf, which is the smallest kind of ...

(PhysOrg.com) -- ESA's XMM-Newton orbiting X-ray telescope has uncovered a celestial Rosetta stone: the first close-up of a white dwarf star, circling a companion star, that could explode into a particular kind of supernova ...

(PhysOrg.com) -- Supernovae are spectacular events: Suddenly somewhere in the heavens a "new star" lights up and shines as bright as a whole galaxy consisting of billions of stars. The mechanisms behind these cosmic catastrophes ...

(PhysOrg.com) -- New findings from NASA's Chandra X-ray Observatory have provided a major advance in understanding a type of supernova critical for studying the dark energy that astronomers think pervades the universe. The ...

(PhysOrg.com) -- An international team led by Yale University has, for the first time, measured the mass of a type of supernova thought to belong to a unique subclass and confirmed that it surpasses what was believed to be ...

Recommended for you

What if I told you that recent experiments have revealed a revolutionary new method of propulsion that threatens to overthrow the laws of physics as we know them? That its inventor claims it could allow us to travel to the ...

The coalescence of two black holes—a very violent and exotic event—is one of the most sought-after observations of modern astronomy. But, as these mergers emit no light of any kind, finding such elusive events has been ...

The recent discovery of an Earth twin has boosted chances there is intelligent life on other planets. But while Pope Francis's telescope scans the starlit skies, the Vatican is sceptical of ever meeting Mr. Spock.

A dying star's final moments are captured in this image from the NASA/ESA Hubble Space Telescope. The death throes of this star may only last mere moments on a cosmological timescale, but this star's demise is still quite ...

in these scenarios does a neutron star behave differently than a white dwarf?

For a neutron star, matter accretion proceeds until the neutron star collapses into a black hole. White dwarfs have far less matter, and matter it accrets normally stays non-degenerate (it is highly compressed normal matter, not neutrons). Only non-degenerate matter has a nucleus that can undergo the fusion necessary to explode in a supernova.

Did all astronomers forget what they learned in Astronomy 101? Unlikely. But the author of this post probably never took it. When hydrogen built up on a white dwarf suddenly ignites, we call it a nova. Lots of astronomers who haven't forgotten about novas are interested in determining whether white dwarfs gain or lose mass during a full nova cycle--they certainly do lose mass while the nova is going on. Unfortunately this is very boring science. Find a few repeating novi. Record their behavior for fifty years or so. Now publish your best guess based on the data. But two papers per century will never get you tenure. :-(

in these scenarios does a neutron star behave differently than a white dwarf?

For a neutron star, matter accretion proceeds until the neutron star collapses into a black hole. White dwarfs have far less matter, and matter it accrets normally stays non-degenerate (it is highly compressed normal matter, not neutrons). Only non-degenerate matter has a nucleus that can undergo the fusion necessary to explode in a supernova.

The authors of these papers are searching for white dwarfs associated with hard X-ray sources, they provide a good catalog of about 100 white dwarfs visible in soft X-rays. (The brightest is Sirius B. Here is a picture of Sirius A & B in X-rays: http://chandra.ha...us.html)

So if Sirius B, a white dwarf with a companion star, is extremely visible in soft X-rays, where are all the other accreting white dwarfs in the Milky Way? Forget about distant galaxies, there should be lots of other white dwarfs in the Milky Way galaxy that stand out due to their X-ray signature.